Semiconductor nanocrystals have been a subject of great interest, promising extensive applications including display devices, information storage, biological tagging materials, photovoltaics, sensors and catalysts. Nanocrystals having small diameters can have properties intermediate between molecular and bulk forms of matter. For example, nanocrystals based on semiconductor materials having small diameters can exhibit quantum confinement of both the electron and hole in all three dimensions, which leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of nanocrystals shift to the blue (i.e., to higher energies) as the size of the crystallites decreases. Semiconductor nanocrystals can have a narrow fluorescence band whose emission wavelength is tunable with the size and material of the nanocrystals.
Nanocrystals consist of an inorganic nanoparticle that is surrounded by a layer of organic ligands. This organic ligand shell is critical to the nanocrystals for processing, binding to specific other moieties, and incorporation into various substrates. Fluorescent nanocrystals are most stable and robust when there is an excess amount of passivating ligands in solution. Monodentate alkyl phosphines and alkyl phosphine oxides passivate nanocrystals efficiently. Note that the term phosphine will refer to both phosphines and phosphine oxides below. Nanocrystals can be stored in their growth solution, which contains a large excess of ligands such as alkyl phosphines and alkyl phosphine oxides, for long periods without noticeable degradation. For most applications, nanocrystals must be processed outside of their growth solution and transferred into various chemical environments. However, nanocrystals often lose their high fluorescence or become irreversibly aggregated when removed from their growth solution.